There is known an air conditioning system in which information is exchanged between an indoor unit and an outdoor unit by serial communication as disclosed in JP-A-08-303842. In some cases, a power supply line which is commonly used between the indoor unit and the outdoor unit is used as a communication line for transmitting serial signals. More specifically, in such an air conditioning system, one of the power supply lines and one signal line independent of the power supply lines are used as a communication line to perform communications.
In the air conditioning system using the power supply line as described above, it is necessary to perform wiring with no error while discriminating a power supply line used as a communication line from the other power supply lines. However, when wiring is performed without the discrimination and thus faulty wiring occurs, some trouble may occur in the communication circuit at the indoor unit side.